It is well known in the art of vehicle design that the fuel consumption of a vehicle associated with its movement is directly related to certain aerodynamic characteristics of the vehicle, such as the aerodynamic drag of the vehicle expressed as the drag coefficient, Cd. As the aerodynamic drag experienced by a vehicle increases, the fuel costs also correspondingly increase due to the greater energy required to overcome the drag. For example, for a vehicle traveling 70 mph on a roadway, approximately 65% of the total fuel consumption of its engine is used to overcome aerodynamic drag. Thus, even a slight reduction in the aerodynamic drag coefficient of the vehicle can result in a significant improvement in fuel economy.
Bluff bodies in particular are known to have high drag coefficients due to the presence of a sizeable recirculation zone in the wake thereof, and the relatively lower pressures acting as a consequence on the rear base of the trailing end. The recirculation zone is formed due to the substantially normal orientation of the base surface with respect to the flowstream, as is commonly seen in tractor-trailer arrangements. This surface orientation creates a sharp separation of the flowstream at the edge of base surface and thereby lowers the pressure on the base surface to produce the base drag. Moreover, for bluff bodies comprising multi-linked or otherwise gap-spaced unit components with gaps between adjacent units, the aerodynamic drag can be even greater when the recirculation zone formed in the gap is disrupted by a cross-flow through the gap. Cross-flow is characterized as a transversely directed flow of air within and/or through the gap due to a pressure difference in a transverse direction across the gap. Cross-flow is especially prevalent when side winds are present which can affect the flowstream characteristics around the bluff body. In such multi-unit or otherwise gap-spaced bluff bodies, an adjacent pair of unit components may be generally characterized as a leading portion and a trailing portion.
Gap cross-flow is often observed with bluff bodied vehicles having towing configurations, such as tractor-trailer arrangements (e.g. having one or more trailers), auto-trailer arrangements, and locomotives, among others. Taking the representative case of a conventional tractor-trailer arrangement, the gap between the tractor and the trailer enables pivoting of one relative to the other. FIGS. 1–4 illustrate such a tractor-trailer arrangement, generally indicated at reference character 100, having a tractor 101 as the leading portion and a single trailer 103 as the trailing portion hitched to and towed by the tractor 101. It is appreciated, however, that conventional tractor-trailer arrangements also include an additional trailer hitched to the first trailer (see for example FIG. 7). In any case, the tractor 101 has a cab portion 102 and a substantially vertical and rear-facing base surface 108. And the trailer 103 has an elongated construction with a front end 104 and a rear end 105. The front end 104 has a forward facing front surface 109 and the rear end 105 has a rear facing base surface 112, with the front surface 109 of the trailer 103 facing the base surface 108 of the tractor 101. A gap 106 is formed between the tractor 101 and the trailer 103, and in particular, between the tractor base surface 108 and the trailer front surface 109.
When placed in a flowstream, such as 107 in FIG. 1, i.e. when the tractor-trailer 100 is in forward motion, the airflow of the flowstream ideally separates off of the tractor 101 and completely reattaches downstream onto the trailer 103. As shown in FIGS. 2 and 3, however, airflow separating from the tractor 101 enters the gap 106 to form a recirculation zone defined by a vortical flow structure 110 which is similar to a vortical ring or an inverted-U shape. A stable vortical flow structure 110 (i.e. one which cannot be forced out of the gap) prevents the surrounding airflow of the flowstream from further entering the gap and thus redirects the surrounding airflow to reattach with the side of the trailer. An unsteadiness in the flow field surrounding the gap, however, can produce a pressure differential in a transverse direction across the gap which can destabilize the vortical flow structure 110 and increase aerodynamic drag. FIG. 4 shows an example of a cross-flow stream 111 completely traversing an empty gap 106 from one side of the tractor-trailer to the other side, through opposing first and second open ends 123 and 124. In this extreme case, the vortical structures would be eliminated altogether by the cross-flow stream 111. However, even small amounts of cross-flow present a compromise in the ability of the vortical structure to prevent airflow from further entering the gap, and can thereby increase the aerodynamic drag on the tractor-trailer 100.
Various methods have been introduced to address this problem of recirculation zone destabilization. One example is shown in U.S. Pat. No. 3,971,586 directed to a drag reducer for land vehicles. As shown in FIGS. 1 and 2 of the '586 patent, the drag reducer is a stabilizer plate 23, mounted on a forward panel 17 of a trailer 16 and extending into a gap 24 in attempting to stabilize vortices 28 and 29 formed in the gap. The stabilizer plate, however, only partially closes the gap, which is an imperfect situation since some air will be forced from one of the divided vortex regions to the other by pressure differences therebetween. By having such an opening through the gap a cross-flow is allowed to form, especially under side wind conditions, which can disturb the vortical structures to adversely impact aerodynamic drag.
U.S. Pat. No. 4,021,069 also shows an apparatus for reducing aerodynamic drag which is for mounting on the bluff, forward face of the trailing element of an over the road vehicle. As can be seen from FIGS. 1 and 2 of the '069 patent, the apparatus is a fairing element mounted at an upper region of the forward face, so as to provide deflection of an impinging air stream. As shown by FIG. 1 in particular, the gap between the tractor and trailer remains substantially unblocked for preventing a cross-flow therethrough.
The need for reducing the aerodynamic drag of bluff body vehicles, especially land-based vehicles traveling at, for example, highway speeds, are compelling and widely recognized. It would therefore be advantageous to provide a simple cost-effective aerodynamic drag reduction apparatus which completely spans the gap to thereby impede cross-flow through a gap between leading and trailing portions of a bluff body, such as a heavy vehicle tractor-trailer, to thereby reduce the net aerodynamic drag on the bluff body.